Air conditioning apparatus

ABSTRACT

Air conditioning apparatus ventilates room air with environmental air after the environmental air has been filtered, heated and humidified to appropriate levels. Environmental air is drawn into an intake duct, is passed successively through an air filter, an electric filament heater and is humidified by directing the air stream through a moving evaporator belt which is saturated with water from a water tank which is automatically controlled by a humidistat. The humidity level and temperature are automatically maintained by means of the humidistat and air temperature sensor.

United States Patent 1191 Follette 1 Mar. 4, 1975 AIR CONDITIONINGAPPARATUS 3,352,353 11/1967 Stevens et 111.... 261/D1G. 4 h I 3,476,36511/1969 Agerley 261/80 [76] Inventor. Donald T. Follett e, 98 SlelghtPlass 3,512077 5/1970 Evalds I I I I 236/78 D poughkeepsle 126033.563.458 2/1971 Martin.. 236/44 A 3684 171 8/1972 Evalds... 236/78 2')1972 3.684172 8/1972 Evalds 236/78 [21] Appl. No.: 284,909

- Primary E.\'aminer-Tim R. Miles 52 us. (:1 261/130, 261/142, 261/80,r. Agent, FirmeBe-mard Malina, q-

236/78, 236/44 C [51] Int. Cl B011 3/04 57 AB TR [58] Field of Search261/80, '142, 130; 1 S ACT 236/44 A, 78, 78 D, 44 C Air conditioningapparatus ventilates room air with environmental air after theenvironmental air has been [56] References Cited filtered, heated andhumidified to appropriate levels. UNITED STATES PATENTS Environmentalair is drawn into an intake duct, is passed successively through an airfilter, an electric 1 filament heater and is humidified by directing theair 1 8/1931 j' 561/80 stream through a moving evaporator belt which issat- 8/1934 Frankel I 261/80 urated with water from a water tank whichis automat- 1.976.-101 10/1934 11g 261/80 ically controlled by ahumidistat. The humidity level 2.0351153 3/1936 Haskell 261/142 andtemperature are automatically maintained by 2.062.042 11/1936 Sargent 1.261/1 2 means of the humidistat and air temperature sensor. 2.074.2653/1937 Kirby 261/DIG. 4 3.339.902 9/1967 Martin 261/80 7 Claims, 5Drawing Figures PAIEIIIEII IR 4% MAX. ALLOWABLE RELATIVE HUMIDITY SHEET3015 RELATIVE HUMIDITY AT WHICH VISIBLE .CONDENSATION WILL APPEAR ONINSIDE SURFACES AT 70F ROOM AIR TEMP.

FIG. 4

(WITH STORM WINDOW) DOUBLE GLASS (WITHOUT STORM WINDOW) SINGLE GLASS 30-20 -IO 0 IO 20 3O 4O OUTSIDE AIR TEMP. "F

I- -o3 Q3 7 I02 87 ZZERO CROSSING I08 I TRIAC DRIVER LT' THERMISTOR ISENSOR 8 9 38 H8 380 38c RI I2O VAC 2 38b| 38,

l AIR CONDITIONING APPARATUS The present invention relates to room airconditioning apparatus and in particular to air conditioning apparatusoperative to automatically render and maintain room air temperature atpreselected temperature and humidity levels.

The equilibrium vapor pressure of any substance increases more and morerapidly as the temperature thereof is increased. The equilibrium vaporpressure of water ranges from 0.1 mm of mercury at -40C (i.e., over ice)to about 55 mm of mercury at 40C. The pressure of water vapor in theair, however, is usually less than the equilibrium value, the existingvapor pressure usually being described by the relative humidity, i.e.,the fraction it forms with the equilibrium value at the existingtemperature. Thus, in a cold climate, air that is saturated or less thansaturated at, for example, F is brought indoors and heated to 75F,whereupon without change in its actual moisture content, its relativehumidity becomes extremely low. Such low humidity is hard on therespiratory passages of the human body, particularly when they double asspeech organs, so that it is important to add moisture to heated roomair in the wintertime.

It is therefore an object of the present invention to provide an airconditioner operative to automatically ventilate, heat and humidify roomair with environmental air to maintain the room air at preselectedtemperature and relative humidity levels.

It is a further object of the present invention to provide an airconditioner of the character just described, which is compact,economical to manufacture and adaptable for easy installation in adouble-hung window.

In accordance with the present invention there is provided an airconditioning apparatus comprising a housing including an air inlet duct,a humidifier chamber, blower means and an air discharge duct. The airinlet duct "has an air inlet part for communication with the ambientatmosphere and an outlet opening communicating with the humidifierchamber. An air filter for trapping dirt particles from the ambientatmosphere is fixedly positioned in the air inlet duct downstream fromthe air inlet port and an electric air heater is fixedly positioned inthe air inlet duct downstream from the air filter. Disposed within thehumidifier chamber is a tank containing liquid and an endlessliquid-transport belt rotatably mounted on a pair of vertically spacedrollers for rotating the belt with flat face thereof parallel to theinlet duct outlet opening. The endless belt has an upper portion facingthe inlet opening and a lower portion immersed in the liquid in thetank.

Electric motor means are connected to at least one of the rollers torotatably transport the endless belt successively into immersion withthe liquid and past the inlet duct outlet opinion to impart moisture tothe air stream exiting through the inlet duct outlet opening. Blowermeans are located downstream of the humidifying chamber for drawingambient air through the air inlet duct and through the liquid transportbelt. An air discharge duct is located downstream of the blower means.Air temperature sensing means and humidity sensing means are located inthe discharge duct for detecting the temperature and humidity of airpassing through the discharge duct. Control means are operative inresponse to the air temperature sensing means and humidity sensing meansfor actuating and deactuating the electric air heater and the electricmotor means driving the rollers to maintain the temperature and humidityof the air stream in the discharge duct respectively at preselectedlevels.

Further objects, features and advantages of this invention will becomeapparent from a consideration of the following description, the appendedclaims and the accompanying drawings in which:

FIG. 1 is a pictorial representation of an air conditioning apparatusconstructed in accordance with the principles of the present inventionin a preferred embodiment thereof;

FIG. 2 is a sectional view of the apparatus of FIG. 1 taken along line2-2;

FIG. 3 is a sectional view taken along the line 3-3;

FIG. 4 is a graph of relative humidity at which visible condensationwill appear on the inside surface of a window pane at F room airtemperature for various environmental air temperature; and

FIG. 5 is an electrical schematic diagram of the air temperaturecontrollercircuit of the present invention.

Referring in detail to the drawings, and in particular to FIG. 1thereof, an air conditioning apparatus constructed in accordance withthe principles of the present invention and for installation in theconventional double-hung window is designated by the numeral 10. Airconditioner 10 comprises an air inlet duct 12 rectangular in crosssection having a downwardly-open throat portion 14 at the forward endthereof for communication with the environmental air outside of thehouse. An air filter 16 is slidably retained in a pair of vertical slots18 and 20 fixedly mounted in sidewalls 22 and 24 of duct 12. Air filter16 comprises a thin rectangular frame 26 enclosing air filter materialsuch as industrial urethane open cell foam 28. A thin rectangularaperture 29 extends transversely across the widthv of duct top wall 30to receive air filter 16 and to allow filter 16 to be slidably receiveddownwardly in filter slots 18 and 20 whereupon the bottom edge of filterframe 26 comes to rest upon duct bottom wall 32. A slot cover 34 forms asubstantially air-tight seal over aperture 29 once air filter 16 hasbeen inserted into duct 12.

An electric filament heater unit 36 is fixedly mounted between duct topand bottom walls 30 and 32, spaced slightly behind air filter 16. Heaterunit 36 comprises a pair of epoxy glass heater frames 33, 35 mountedback-to-back in an aluminum frame 37. Each of epoxy glass frames 33, 35is divided into three openings to form three sections 33a, 33b, 33c,35a, 35b, 35c. Approximately 10 feet of 0.010 inch diameter nichromeheater wire 38 having a resistance of about 6.66 ohms/ft is strungacross the frame openings on metal standoffs located along the epoxyglass heater frames 33, 35 to form three heater screens 38a, 38b, 380 offairly small mesh interposed across the air flow path, to form a totalresistive load of 11 ohms.

Air inlet duct 12 opens into a humdifying chamber 46 bounded bysidewalls 48 and 50 extending from duct sidewalls 22 and 24respectively. A water tank 52 in the form of abox-like housing 54extends below humidifying chamber 46 is partially filled with water 56and is provided with a water level gauge 55. Water tank housing 54comprises front and rear walls 58, 60, bottom wall 62 and sidewalls 64and 66. An endless evaporator belt 68 is mounted on top and bottomrollers 70, 72 which are horizontally mounted in a separate frame 2llocated in humidifier chamber '46, with top roller 70 mounted across theupper ends of the frame sidewalls 21a and 21b and bottom roller 72mounted across the lower ends thereof which is immersed in the water 56.The provision of a separate belt frame 21 permits the removal thereoffrom humidifier chamber 46 for cleaning or replacement. A suitablematerial for belt 68 may be that known as industrial urethane open cellfoam material similar to air filter material 28 but of larger void area.

Water tank housing 54 is closed at its upper end by means of a top wall74 which defines a belt opening 76 between the free edge thereof and therear end of duct bottom wall 32.

An electrical air blower 88 which comprises a centrifugal fan 89 withtangential blades 90 is mounted in blower housing 86 which includes ascroll-shaped duct wall 92 tangentially fixed to tank top wall 74 andblower housing front wall 84. Blower housing 86 at its front end opensinto humidifier chamber 46 and into discharge duct 82 at its upper end.

Discharge duct 82 is pivotally mounted to front wall 84 of blowerhousing 86 by means of hinges 101 whereby discharge duct 82 can bepivoted to the open position depicted by phantom outline 103 to permitaccess to the interior thereof and blower housing 86. A baffle 99 isprovided to direct the air flow produced by blower 88 into dischargeduct 82.

Humidifier chamber .46 is bounded at its top end by the underside ofdischarge duct wall 78 and by front wall 80 which includes a shoulderportion 80a for supporting duct wall 78. Duct wall 78 terminates at itslower end in an angle portion 49 which rests on and overhangs the topedge of blower housing wall 51.

An air temperature sensor 87 in the form of a thermistor is located indischarge duct 82 just below grilled air discharge outlet 96. A controlpanel 90 provided with control buttons and dials for setting the desiredair quality characteristics is secured to front wall 93 of dischargeduct 82. Thus, dial 92 sets the humidity level, dial 94 sets the speedof evaporator belt 68, buttons 95 control air inlet shutter assembly 85which controls the movment of air inlet port shutter 15 via cablelinkage 109, dial 98 sets the desired air temperature at dischargeoutlet 96 and indicator light 97 indicating whether evaporator belt 68is moving or has stopped. The electronic sensing and control circuitryfor accomplishing the functions controlled by dials 92, 94, 98 arelocated in a rectangular housing 100 which is secured to the inside ofduct front wall 93.

In operation, unit is installed in a room window by resting theunderside of inlet duct 12, i.e., bottom wall 32, on the window sash(not shown) and allowing water tank housing 54 to overhang the windowsill along the wall adjacent to the window. Bottom wall 32 is providedwith a shoulder 17 to clear the storm window frame (not shown) which iscommonly found in windows. When electrical power is applied to blower88, a negative air pressure, i.e., suction pressure, is created in inletduct 12, and humidifying chamber 46, Le, upstream from blower 88,thereby drawing environmental air into air inlet port 13. Air inlet duct12 is provided with air inlet port shutter to control the flow ofenvironmcntal air into inlet duct 12, and which as mentioned above isopened and closed manually by means of a cable linkage 85.

The air flow, whose path is indicated by dashed line 19, continues fromthe mouth of inlet port 13 through duct 12 and through air filter 16where the air filtration 1 Le, its ability to overcome the airflowimpedance path through air filter foam material 28.

The location of air filter 16 upstream from heater unit 36 andhumidifier chamber 46 is significant in that air borne dust or dirtparticles are thereby removed from the air stream before the air reachesthe heating elements of heater unit 36 and evaporator belt 68.Otherwise, air-borne dirt particles in the air stream would tend to foulthe heating elements 38 and would also cause the creation of unpleasantodors in addition to increasing the resistance of the heat transfer pathfrom heating elements 38 to the air stream. Because the air filtermaterial 28 comprises open cell foam material it can be easilycleaned byusing an ordinary household portable vacuum cleaner after removingfilter 16 from air inlet duct 12, without disturbing the installation ofunit 10.

After passing through air filter 16, the air flow continues throughheater unit 36 which is operative to raise the temperature of the airpassing therethrough as measured by air temperature sensor 87 to atemperature corresponding to the setting of air temperature dial 98. Theair stream then passes through evaporator belt 68 which is saturatedwith water that has been picked up as belt 68 passes through the water56 in water tank 52.

The elements of air conditioner 10 which accomplish the air heating andtemperature control of the air stream are respectively heater unit 36and the proportional controller which includes air temperature sensingthermistor 87. The proportional controller may advantageously constitutea phase voltage firing controller 83 which is operative to maintain arelatively fixed discharged air temperature at discharge outlet 82within the temperature range available at temperature control dial 98,typically 60F 75F, regardless of changes in either the environmental airtemperature or the flow rate of the air stream. Thus, by way of example,for a fixed air flow rate of 30 40 CFM, the proportional controller canmaintain a discharge air temperature of F for environmental airtemperatures ranging from 70F -20F. It is appreciated that instead ofthe phase voltage firing controller just described, a zero voltagefiring proportional controller may be utilized, which although notcapable of maintaining the discharge air temperature as precisely as thephase voltage firing controller, may nevertheless be desirable whereradio frequency interference may be a serious problem.

Continuous air humidification sensing and control for the air flow inhumidifier chamber 46 is accom plished in the following manner.Humidification sensing of the air stream in discharge duct 82 may beaccomplished by using materials which exhibit a change in some parameterthereof which has a known relationship with the relative humidity levelof the air stream. Such materials are of two kinds, firstly thedimensional type which are particularly suitable for on-off control ofthe parameter which governs the evaporation rate,

and secondly, material which when coupled with a displacement transducerwhich produces a continuous electrical output for the continuous controlof the evaporation rate parameter, i.e., the relative humidity level.The latter electrical parameter change type materials are best suitedfor the continuous or proportional type control of the relative humiditycontrol parameter, in which case, signal converters are ordinarilyrequired to convert the output of the humidification sensor to anelectrical signal suitable for control purposes.

The nylon tape in humidistat 81 expands and contracts in response to anincrease and decrease respectively in the humidity of the air passingtherethrough. The expansion and contraction of the nylon tape isperative in response to a particular humidity setting of humidistatcontrol dial 92, to actuate the on-off switch (not shown) which supplieselectrical power to evaporator belt drive motor 69.

The principles of the present invention described herein are based uponthe use of an on-off humidity sensing and control system. Thus, anon-off type of humidistat 81 which comprises a humidity-sensor 79 ofnylon tape may be employed to control the amount of moisture to beimparted to the air stream in humidifier chamber 46 in conjunction witha graph showing maximum allowable relative humidity vs. environmentalair temperature such as the graph shown in FIG. 4 to determine themaximum allowable relative humidity for the air discharged fromdischarge outlet 96. A suitable humidistat 81" for such purpose is theHumidity Controller Type 46B1 192-2 sold by Honeywell, Inc.

The operating procedure of unit 10 will now be described for the case ofovernight use. The user, having determined the approximate overnight lowof the environmental air temperature, examines the maximum allowablerelative humidity reading from the abovementioned graph of maximumallowable relative humdity vs. environmental temperature curve. He thensets humidistat control dial 92 to the relative humidity levelcorresponding to the environmental air temperature and sets belt speedcontrol dial 94 to its maximum setting. Power to unit 10 is themprovided by actuating unit on-off switch 57 and belt drive onoffindicator light 97 is observed. If indicator light 97 remainscontinuously on, this would be an indication that the humidificationprocess is operating at its maximum capacity and that the relativehumidity of the discharge air is not greater than that set by humidistatcontrol dial 92. If, however, indicator light 97 lights upintermittently, this would indicate that the relative humidity of thedischarge air is greater than the relative humidity setting ofhumidistat control dial 92, calling for manual correction by reducingthe speed of evaporation belt 68 by adjusting dial 94 in successivesmall steps until belt drive indicator light 97 remains continuously on,indicating that the discharge air relative humidity has not exceeded thesetting of humidistat control dial 92.

Referring to FIG. the air temperature controller circuit for controllingthe temperature of air flow in discharge duct 82 is shown in schematicform. The three heater screens 38a, 38b and 380 appear as six resistiveelements 38:1 380 3811,, 38b 380 and .38c 'connected in parallel with atotal resistive load of 11 ohms. This resistive circuit 38 is connectedat one end to one electrode 104 a Triac 102 which has another electrode106 thereof jointly connected to zero-crossing Triac drive circuitterminals 4 and 5 and a gate electrode 108 connected to terminal 7 ofTriac driver circuit 83. Thermistor 87 comprises one leg of a bridgecircuit including resistors, 110, 112, 114, 116 and 118 whereby thejunction of thermistor 87 and resistor is connected to terminal 2 ofTriac driver circuit 83, the junction of resistors 112 and 116 isconnected to terminal 1, and the junction of resistors 114 and 118 isconnected to terminal 3 of Triac driver circuit 83. Additionally acapacitor 120 is connected across resistor 110 and thermistor 87, acapacitor 122 is connected across terminals 6 and 8 of Triac drivercircuit 83 and resistor 124 is connected between terminal 6 of Triacdriver circuit 83 and the other end of resistive heater circuit 38. Theair temperature controller circuit of FIG. 5 is connected to a standard120 volts A.C. power source by leads 126 and 128 which are respectivelyconnected to terminals 4 and 5 of Triac driver circuit 83 and the otherend of heater circuit 38.

In operation, Triac driver circuit 83 is operative to compare thevoltage produced across thermistor 87 corresponding to the measured airtemperature with a preselected voltage corresponding to the desired airtemperature as determined by air temperature control dial 98. If thethermistor voltage is less than the reference voltage, a control voltagesignal is produced at Triac driver circuit terminal 7 which is appliedto gating electrode 108 of Triac 102 causing Triac 102 to fire andthereby connect heater circuit 38 to power source 130 to cause heater 38to heat the air stream in discharge duct 82. The resultant increase inthe tempera ture of the air flow is discharge duct 82 is reflected in acorresponding increase in the voltage across thermistor 83 and when theair temperature increases to above the preselected level the voltagesignal terminal 7 of Triac driver circuit 83 ceases and Triac 102presents an open circuit across its electrodes 104 and 106 therebydeenergizing heater circuit 38.

In addition to using unit 10 on a continuous basis as just described,unit 10 may be employed for short periods of time in a conventionalhumidifier mode for the purpose of raising the relative humidity levelof the room air just prior to ventilating, by making slightmodifications (not shown) in the design of unit 10. For such operation,a second inlet port would be provided just upstream of air filter 16 aswell as a two-speed motor (not shown) for driving blower 88. In thismode of operation, humidistat dial 92 and evaporator belt dial 94 areset to their maximum settings respectively. Air inlet port 14 is closedwhile the above-mentioned second inlet port (not shown) is opened andblower 88 is turned on to its high speed operation.

Although the invention has been described with reference to particularembodiments thereof, it is to be understood that such embodiments aremerely illustrative of the application of the principles of theinvention. Numerous modifications may be made therein and otherarrangements may be devised without departing from the spirit and scopeof the invention.

I claim:

1. Air conditioning apparatus comprising a housing including an airinlet duct, a humidifier chamber, an air discharge duct and enclosingblower means for drawing air through said air inlet duct, saidhumidifier chamber and said air discharge duct, said air inlet ducthaving an air inlet port for communication with the ambient air and anoutlet opening communicating with said humidifier chamber, an air filterfor trapping dirt particles present in the ambient air fixedlypositioned in said air inlet duct downstream from said air inlet port,an electric air heater fixedly positioned in said air inlet ductdownstream from said air filter, a tank containing liquid disposedwithin said humidifier chamber, an endless liquid-transport beltrotatably mounted on a pair of vertically spaced rollers for rotatingsaid belt with the flat face thereof parallel to said inlet ductopening, said endless belt having an upper portion thereof facing saidinlet duct outlet opening and having a lower portion thereof immersed insaid liquid, electric motor means connected to at least one of saidrollers whereby said motor means when actuated drives said rollers torotatably transport said endless belt successively into immersion insaid liquid and past said inlet duct outlet opening to impart moistureto the air stream exiting through said inlet duct outlet opening, blowermeans located downstream of said humidifying chamber for drawing ambientair through said air inlet duct and through said liquid-transport belt,an air discharge duct located downstream of said blower means, airtemperature sensing means and humidity sensing means located in said airdischarge duct for detecting the temperature and humidity of air passingthrough said discharge duct, control means operative in response to saidair temperature sensing means and humidity sensing means for actuatingand de-actuating said electric air heater and said electric motor meansdriving said rollers to maintain the temperature and humidity of the airstreamv in said discharge duct respectively at preselected levels, ashutter pivotally mounted in said air 8 inlet port for opening andclosing said air inlet port, shutter control means on the front face ofsaid housing and a cable linkage connecting said shutter to said shuttercontrol means whereby actuation of said shutter control means isoperative to selectively open and close said air inlet port.

2. Apparatus as defined in claim 1 wherein said liquid-transport beltcomprises urethane open cell foam material.

3. Apparatus as defined in claim 1 wherein said blower means comprises atangential blower having its intake communicating with said humidifyingchamber and its outlet communicating with said air discharge duct.

4. Apparatus as defined in claim 1 wherein said air temperature sensingmeans comprises a thermistor.

5. Apparatus as defined in claim4 wherein said humidity sensing meanscomprises nylontape.

6. Apparatus as defined in claim 4 wherein said control means comprisesa resistor bridge circuit including said thermistor, gating means havingone of its main electrodes connected to said electric heater and anotherof its main electrodes adaptable for connection to an AC. power source,and a gating means driver circuit having its input connected to saidresistor bridge circuit and to the other of said gating means mainelectrodes, said driver circuit having its output connected to the gateelectrode of said gating means.

7. Apparatus as defined in claim 6 wherein said gating means comprises atriac.

1. Air conditioning apparatus comprising a housing including an airinlet duct, a humidifier chamber, an air discharge duct and enclosingblower means for drawing air through said air inlet duct, saidhumidifier chamber and said air discharge duct, said air inlet ducthaving an air inlet port for communication with the ambient air and anoutlet opening communicating with said humidifier chamber, an air filterfor trapping dirt particles present in the ambient air fixedlypositioned in said air inlet duct downstream from said air inlet port,an electric air heater fixedly positioned in said air inlet ductdownstream from said air filter, a tank containing liquid disposedwithin said humidifier chamber, an endless liquid-transport beltrotatably mounted on a pair of vertically spaced rollers for rotatingsaid belt with the flat face thereof parallel to said inlet ductopening, said endless belt having an upper portion thereof facing saidinlet duct outlet opening and having a lower portion thereof immersed insaid liquid, electric motor means connected to at least one of saidrollers whereby said motor means when actuated drives said rollers torotatably transport said endless belt successively into immersion insaid liquid and past said inlet duct outlet opening to impart moistureto the air stream exiting through said inlet duct outlet opening, blowermeans located downstream of said humidifying chamber for drawing ambientair through said air inlet duct and through said liquid-transport belt,an air discharge duct located downstream of said blower means, airtemperature sensing means and humidity sensing Means located in said airdischarge duct for detecting the temperature and humidity of air passingthrough said discharge duct, control means operative in response to saidair temperature sensing means and humidity sensing means for actuatingand de-actuating said electric air heater and said electric motor meansdriving said rollers to maintain the temperature and humidity of the airstream in said discharge duct respectively at preselected levels, ashutter pivotally mounted in said air inlet port for opening and closingsaid air inlet port, shutter control means on the front face of saidhousing and a cable linkage connecting said shutter to said shuttercontrol means whereby actuation of said shutter control means isoperative to selectively open and close said air inlet port. 2.Apparatus as defined in claim 1 wherein said liquid-transport beltcomprises urethane open cell foam material.
 3. Apparatus as defined inclaim 1 wherein said blower means comprises a tangential blower havingits intake communicating with said humidifying chamber and its outletcommunicating with said air discharge duct.
 4. Apparatus as defined inclaim 1 wherein said air temperature sensing means comprises athermistor.
 5. Apparatus as defined in claim 4 wherein said humiditysensing means comprises nylon tape.
 6. Apparatus as defined in claim 4wherein said control means comprises a resistor bridge circuit includingsaid thermistor, gating means having one of its main electrodesconnected to said electric heater and another of its main electrodesadaptable for connection to an A.C. power source, and a gating meansdriver circuit having its input connected to said resistor bridgecircuit and to the other of said gating means main electrodes, saiddriver circuit having its output connected to the gate electrode of saidgating means.
 7. Apparatus as defined in claim 6 wherein said gatingmeans comprises a triac.